1. Field of the Invention
The present invention relates to the field of data communications. More particularly the invention describes using a zero-overhead method for sequence reversible and pattern independent orthogonal multiplexing to achieve high bit densities in direct sequence spread spectrum communication systems.
2. Description of Related Art
Direct Sequence Spread Spectrum (DSSS) techniques rely on the use of pseudo-noise carriers, also called spreading codes, spreading sequences, code sequences and chip sequences, and a transmission bandwidth, which is much wider than the minimum required to transmit the information. The transmitter spreads the information by modulating the information with a pseudo-noise spreading sequence. At the receiver, the information is despread to recover the base information. This despreading is accomplished by correlating the received, spread-modulated, signal with the spreading sequence used for the transmission. DSSS is sometimes referred to by the shorthand name “direct spread.”
The modulating signal, such as a pseudo-random spreading code signal, possesses a chip rate (analogous to carrier frequency), which is much larger than the data rate of the information signal. This characteristic is required for efficient spreading. Each state of the pseudo-random spreading sequence is referred to as a chip. The spreading sequence (chip sequence) directly modulates each bit of the information signal, hence the name direct spread. Pseudo-randomness of the spreading signal is required in order to recover the original information signal. Since the spreading sequence is deterministic, it can be exactly duplicated at the receiver in order to extract the information signal. If it were truly random, extraction of the information signal via correlation receiver would not be possible.
The spreading operation causes the signal power to be depleted uniformly across the spread bandwidth. Thus, the spread spectrum signal will appear buried in noise to any receiver without the despreading signal. Consequently, it is not only difficult to jam, but it is also difficult to detect its presence in any bandwidth. Any undesired signal picked up during transmission is spread by the receiver in the same way that the transmitter spread the desired signal originally. In other words, the receiver spreads undesired signals picked up during transmission, while simultaneously despreading, or demodulating, the desired information signal. When viewed as a transmit/receive operation, the desired signal is spread-modulated twice, giving back the original signal, while in-band interference is spread-modulated once, and thereby depleted across the full spread bandwidth.
Similarly, Direct Spread Code Division Multiple Access (DS CDMA) is an adaption of conventional DSSS, which accommodates multiple simultaneous access through the use of mutually orthogonal spreading codes. Mutually orthogonal means that the cross correlation of any spreading code in the set is small (ideally zero). In an ideally orthogonal system, orthogonality not only means that there is no mixing of signals, but it also means that there is no interference between signals. In a conventional DS CDMA system, each station is assigned a single spreading code, which conveys one bit of information. All users assigned to a given channel broadcast either a true or complement version of their assigned spreading code simultaneously. Since the codes are orthogonal, each user's spreading code is uniquely decoded by the receiver. The multiple-access is via the orthogonal properties of the spreading code, hence the name CDMA. Like the previously described DSSS systems, stations only transmit one bit of information at a time.
Thus, in conventional DSSS and DS CDMA systems, an input binary data stream is transmitted one bit at a time, and each bit is modulated with a spreading code, wherein each successive transmission is independent of prior transmissions. Furthermore, the spread modulation can occur in either the frequency domain (i.e. by frequency spreading) or the time domain (i.e. by time spreading). Unfortunately, both methods (frequency and time spreading) incur high penalties for transmission efficiency by trading off efficiency for high redundancy. Typically, conventional DSSS and DS CDMA techniques only provide a bit encoding density of (1/N)*100%, where N is the spreading code length.